Historically and across cultures, marriage has been the institution that unites a man and woman as husband and wife to be father and mother to children born of their union, providing their offspring with the distinctive contributions of paternal and maternal care and influence. That understanding of marriage shaped its structuring norms, including the norm of sexual complementarity that has been found always and everywhere.

Yesterday this understanding of marriage was common sense; today it is dangerous heresy to an ever-more Inquisitorial cultural elite: Support for marriage as a male-female union has swiftly become, among the mandarins of culture, the most hated position on the most heated question.

In April, more than 50 scholars and leaders, all self-identified same-sex marriage supporters, called their allies to higher moral ground. Prompted by the bullying of Brendan Eich and his resignation as CEO of Mozilla for his 2008 donation to California's Proposition 8 campaign, they wrote to decry the "deeply illiberal impulse" to "punish rather than to criticize or to persuade" political dissenters. Because "opposition to gay marriage" can be "expressed respectfully," they urged, it should not be "a punishable offense." No one should lose a job for "holding a wrong opinion." Trying to purge the workplace or the public square of dissent is, as they see it, political "oppress[ion]."

As supporters of marriage as the union of husband and wife, we applaud the signatories' support for a free society. In any healthy civic order, citizens will be able to disagree with each other even about important matters without intimidation and recrimination. The right to dissent will be honored and those who express dissent will be respected not bullied into submission or silence. We thank the signatories of "Freedom to Marry, Freedom to Dissent" for their defense of civility.

We also agree with them that the future of marriage needs to be a matter of robust public debate. That's why it's important for these and other supporters of same-sex marriage -- indeed for all Americans -- to engage the serious arguments in this debate.

We write to make three points.

1. It is rational to support marriage as the union of a man and a woman, and supporters of same-sex marriage should stand up and say so, condemning attempts to disparage belief in marriage as a conjugal partnership as irrational -- the moral and intellectual equivalent of racism, misogyny, and other forms of bigotry.

The statement issued last month falls short of such a declaration, lacking a clear acknowledgment that those with the contrary view hold a rationally defensible position.

The signatories of "Freedom to Marry, Freedom to Dissent" do not go so far as to acknowledge the merit in arguments against same-sex marriage. ... their public statement indicates that in their view their opponents have little to say that is reasonable. Unwavering in their commitment to same-sex marriage, they imply that those who disagree are at best benighted, ignorant, or confused.

Indeed, by suggesting that support for marriage as a union of husband and wife is simply opposition to the cause of "gay equality," the signatories seem to share -- and certainly say nothing to reject -- the idea that opposition cannot be rational; that only ignorance or animus could motivate it; that it is ultimately a matter of bigotry.

Yet these are precisely the false and pernicious ideas that motivated the persecutors of Brendan Eich. In fact, Eich's case is nothing new. For years, lawmakers and cultural leaders have treated supporters of marriage as it has always existed as irrational and bigoted -- as enemies of basic human rights. And in this they were only taking the movement for "marriage equality" at its word. They were giving effect to a premise that many leading marriage-redefinition advocates have long championed. In criticizing the recent statement, Jonah Goldberg has observed:

"If you've spent a decade or more advancing the argument that opposition to same-sex marriage is morally equivalent to opposition to interracial marriage and other Jim Crow laws, it shouldn't come as too much of a surprise when people who bought your argument then go out and apply its ruthless and unforgiving logic."

Some of the signatories have always rejected the equating of conjugal marriage laws with laws against interracial marriage; perhaps others now regret ever drawing that equivalence -- though, if they do, they ought to say so. Indeed, all supporters of same-sex marriage should now repudiate it. Failing that, those who embrace the arguments for same-sex marriage are likely to continue trying to stigmatize their opponents in an effort to drive them to the margins of public life where our culture rightly deposits its haters and bigots.

2. The government should not discriminate against or coerce those who speak and act on the belief that marriage is the union of a man and a woman. Policy should protect the rights of individuals and groups to honor their conscientious judgments on the central question in the debate: What is marriage?. The statement denounces the bullying of Brendan Eich even though that bulling involved no "formal legal sanction." Yet legal sanctions have already been used to coerce people and institutions to violate their conscientious judgments about the nature of marriage. Citizens have not only a right to hold and voice opinions about marriage, but also a right to live by those views.

For years, a central argument of those in favor of same-sex marriage has been that all Americans should be free to live as they choose. But does that freedom require the government to coerce people, against their consciences, into participating in or facilitating celebrations of same-sex relationships? In a growing number of incidents, the redefinition of marriage and state policies adding sexual orientation and gender identity (SOGI) to the lists of protected classes under non-discrimination statutes have led to the intolerance and intimidation of citizens who believe that marriage unites a man and a woman and that sexual relations are properly reserved for marriage. Same-sex marriage advocates, it seems, were in many cases engaged in a deception when they claimed that their causes was one of "live and let live."

Now citizens who persist in dissent from the new elite orthodoxy are facing a new wave of government coercion and discrimination. State laws regarding sexual orientation and gender identity are being used to trump civil liberties such as freedom of speech and the free exercise of religion.

Consider the case of Barronelle Stutzman, the owner of Arlene Flowers. In March 2013, longtime customers of hers asked her to arrange flowers for their same-sex wedding ceremony. As a matter of conscience, Stutzman had to decline because of her belief that marriage is between a man and a woman. While she was happy to arrange and sell them flowers for any other occasion, as she had for nine years, she could not in conscience use her artistic skills to help celebrate a same-sex ceremony.

A month later, Washington attorney general Bob Ferguson filed suit against Stutzman, contending that she had violated the state's sexual-orientation antidiscrimination law. The state of Washington is seeking a $2,000 fine and a court order forcing Barronelle to prepare such arrangements against her conscience. In his zeal for the same-sex marriage cause, Ferguson has decided to use the power of the state to crush an opponent -- treating her precisely as if her views make her the equivalent of a racist in the Jim Crow south.

Stutzman is not the only small-business owner whose religious liberty is at risk. Families across the country are being hauled into court for living by their belief that marriage is the union of a man and a woman. Cake makers, photographers, family bakeries, and adoption agencies, among others, have faced penalties and lawsuits or been driven out of business for working in accordance with their faith.

State and federal policy must respect Americans' ability to live and work in keeping with their beliefs about marriage. While Americans are free to live as they choose, no one should be compelled against conscience to facilitate a same-sex sexual bond. Opposing such sexual partnerships on conscientious moral grounds is not the equivalent of racism. And yet last month's open letter completely ignored this concern.

3. The terms "marriage equality" and the "freedom to marry" sidestep the central issue of what marriage is.

Appeals to "marriage equality" are good sloganeering, not good reasoning. Every law -- every marriage policy -- draws lines. Equality before the law protects citizens from arbitrary line drawing, from laws that treat them differently for no good reason. To know whether a law makes the right distinctions -- whether the lines it draws are justified -- one has to know its public purpose and the nature of the good that it serves.

Now that, in many states, the law is treating same-sex couples as spouses, will some people argue that it still fails to respect the equality of citizens in multiple-partner relationships? Are those inclined to such relationships being treated unjustly -- do they lack freedom and equality, and are they and their children stigmatized -- when government does not recognize their consensual romantic bonds as marriage and allow them to file taxes jointly?

It's not just that the phrases "marriage equality" and "freedom to marry" are elastic; some are using such flexible thinking about marriage to push its redefinition beyond just couples. This is not merely hypothetical. In 2009, Newsweekreported that there were over 500,000 polyamorous households in America. Prominent scholars and leading LGBT activists have publicly called for legal recognition for multipartner relationships since at least 2006. What basis of principle could distinguish their claim from that of the same-sex couple?

A certain type of polyamorous relationship has even motivated advocates to create the word throuple, which is similar to "couple" but involves three people. The word appeared in a 2012 article in New York magazine. And in late April a New York Post headline read: "Married lesbian 'throuple' expecting first child." It is not clear why "marriage equality" and the "freedom to marry" do not require recognizing their bond. Based on what principle, consistent with a denial that marriage is the conjugal union of husband and wife, can one refuse to deem their relationship a marriage?

As we note in our book, What Is Marriage? Man and Woman: A Defense, we have yet to see a plausible argument that explains why governments can treat these bonds differently, without depriving them of "marriage equality" or the "freedom to marry." Once you say defining marriage as the union of a man and woman is irrational and arbitrary, solely the result of animus as Justice Kennedy suggested, what is magical about twosomes? Monogamy in Western law and culture reflects the fact that it takes a man and a woman to create a new life, and every new life has one mother and one father. Marriage is the relationship that unites these people in a permanent and exclusive union. But those suggesting the male-female aspect is arbitrary have yet to provide an argument for why monogamy, exclusivity, or permanency must remain. We and others have posed such challenges for years. Some prominent marriage revisionists -- from Gloria Steinem to Kenji Yoshino -- have effectively conceded those challenges, accepting the consequence that we all must treat these other consensual bonds as marriages or the equivalent.

Answering the question "What is marriage?" would go a long way toward advancing the discussion to which both signers of last month's letter and we remain committed. And it would be a powerful acknowledgment of the complexity of this issue -- of why it is one on which decent people disagree, and toward which political punishment is oppression.

Ryan T. Anderson is the William E. Simon fellow at the Heritage Foundation. Robert P. George is McCormick professor of jurisprudence at Princeton University. Along with Sherif Girgis they are co-authors of What Is Marriage? Man and Woman: A Defense.